The Copenhagen interpretation

In 1921, Niels Bohr (1885-1963) founded the Institute of Theoretical Physics in Copenhagen to develop the mechanics for dealing with the world of the ultimate constituents of matter — atoms, atomic nuclei and photons of light. The most eminent physicist from all over the world attended Bohr's institute at one time or another, and the results of their deliberations became known as the Copenhagen interpretation of quantum mechanics. It soon became clear that the wave-particle duality of light is a symptom of a much deeper principle at the core of the laws of Nature. In the atomic world physical entities exist in superposition of states. For example, an atom may exist as a mixture of states of different energies, and acquires a given energy state only when energy is measured. A photon is neither a particle nor a wave but acquires one or other identity when it is observed. We have to revise our understanding of reality. In the world of atoms, molecules and light quanta, physical objects do not have an independent existence.

It is somewhat ironic that Albert Einstein, who had been the first to approve de Broglie's particle-wave hypothesis and had later made his vital contribution to the quantum theory of the photoelectric effect, became one of the greatest critics of the Copenhagen interpretation of quantum mechanics. He could not come to terms with the suggestion that physical attributes of particles and systems depend on what measurements are made, or whether or not measurements are made at all. How could the act of observation change physical reality? In one of his many letters to Bohr he expressed what was to him the absurdity of it all: "How could a mouse change the universe by looking at it?" Einstein's own theory of relativity was derived by logical steps from initial logical assumptions about space and time and the constancy of the speed of light. To him 'the most incomprehensible thing about Nature was that it is comprehensible'. The philosophy of quantum mechanics appeared to be neither logical nor comprehensible!

Einstein developed the theory of relativity on the basis that the speed of light is a universal constant. In empty space, there are no 'milestones' and therefore no reference points to define absolute speed. This was the starting point of Einstein's logical train of thought which led to his famous equation E = mc2.